docs: def _interp_function(self, y_ip1, y_i, t_ip1, t_i, imt_per):
          """
          Generic interpolation function used in equation 19 of 2013 report.
          """
          return y_i + (y_ip1 - y_i) / (t_ip1 - t_i) * (imt_per - t_i)